In many scientific or industrial applications, compounds are purified for testing, analysis, or volume production. Purification of a compound involves separating a desired compound from a mixture that contains additional compounds and/or impurities.
Chromatography is a method for fractionating a mixture to separate compounds of the mixture, and at times is used for purification. In liquid chromatography, for example, a sample containing a number of compounds to be separated is injected into a fluid stream (i.e., a solvent,) and directed through a chromatographic column. The column separates the mixture into its component species in response to differential retention of the component species in the column. Concentration peaks associated with the separated compounds typically emerge in sequence from the column.
The chromatographic peaks are often characterized with respect to their retention time, that is, the time in which the center of the band transits the detector relative to the time of injection. In many applications, the retention time of a peak is used to infer the identity of the eluting analyte based upon related analyses incorporating standards or calibrants. The presence of the separated species are often distinguished through use of a refractometer or an absorbtometer utilizing ultraviolet (UV) light.
A typical high-performance liquid chromatography (HPLC) system includes a pump for delivering a fluid (a “mobile phase”) at a controlled flow rate and composition, an injector to introduce a sample solution into the flowing mobile phase, a tubular column encasement containing a packing material or sorbent (a “stationary phase”), and a UV detector to register the presence and amount of the sample compounds in the mobile phase. The presence of a particular compound in the mobile phase exiting the column is then detected by measuring changes in physical or chemical properties of the eluent. Response peaks corresponding to the presence of each of the compounds of the sample can be observed and recorded by tracking the detector's signal over time.
For target purification, it is often desirable to recover the target compound with as high a purity as possible, and to separate the largest possible quantity of a sample with each run to reduce labor, run time, and other costs. Typical chromatography systems, however, have sample load limits. In some cases, use of overloaded samples provides increased processing volumes. Overloading, however, typically involves increased complexity in extraction of a target compound through use of a displacer material or a segmented column. Moreover, the extracted target often has less than a desired purity.